1. Field of the Invention
The present invention relates to a coating composition capable of forming a hard coating film that combines abrasion resistance and crack resistance, as well as a coated article that uses this composition.
2. Description of the Prior Art
Examples of known coating compositions used for forming surface-protective coatings on the surfaces of organic resin substrates such as plastics with the aim of imparting superior levels of hardness and abrasion resistance include compositions obtained by hydrolysis or partial hydrolysis of a hydrolyzable organosilane, and compositions obtained by mixing such a composition with a colloidal silica.
For example, patent reference 1, patent reference 2, patent reference 3 and patent reference 4 disclose coating agents composed of an organoalkoxysilane, a hydrolysis product of the organoalkoxysilane and/or a partial condensation product thereof, and a colloidal silica, wherein the alkoxy groups are converted to silanol groups in an excess of water. However, although the coating films obtained from these coating agents exhibit a high degree of hardness, favorable weather resistance and excellent substrate protection, they lack toughness. For example, if a coating film with a thickness of 10 μm or greater is removed from the curing oven during heat-curing, and subsequently used outdoors, then the coating film is prone to cracking upon rapid variations in temperature. Furthermore, even though a buffered basic catalyst is used as the curing catalyst in order to ensure more favorable storage stability, because the hydrolysis/condensation product of the alkoxysilane within these coating compositions is composed mainly of comparatively low molecular weight substances, and because the reactivity of the silanol groups incorporated within these comparatively low molecular weight substances is extremely high, and the silanol content is high, these low molecular weight substances undergo gradual condensation even at room temperature, causing an increase over time in the molecular weight that leads to a decrease in the coating hardness. Gelling of the coating may also occur in some cases, and these problems of stability mean practical application of these coating agents is impossible.
In order to address these problems, patent reference 5 proposes a composition which, by using a specific basic compound as a curing catalyst, combines favorable storage stability for the liquid, with superior levels of crack resistance, hardness and abrasion resistance for the coating film. Moreover, in patent reference 6, by including a component for imparting flexibility as an essential component, and setting the weight average molecular weight for the organosiloxane resin to a comparatively high value of 1,500 or greater, a combination of abrasion resistance and crack resistance can be achieved to some extent, although the composition is unable to prevent the occurrence of micro cracks in an extended weather resistance test, meaning the durability is not entirely satisfactory.
[Patent Reference 1] GB 1454960 A
[Patent Reference 2] GB 1596151 A
[Patent Reference 3] GB 2001870 A
[Patent Reference 4] JP 63-168470 A
[Patent Reference 5] US 2005/0244659 A1
[Patent Reference 6] US 2006/0083936 A1